1. Field of the Invention
The present invention relates to a polishing agent and a polishing method used for polishing semiconductor silicon wafers (hereinafter may be simply referred to as wafer, semiconductor wafer or silicon wafer), which can effectively prevent metal contamination, particularly Cu and Ni contamination of semiconductor silicon wafers in a polishing process.
2. Description of the Related Art
Generally, a method of manufacturing semiconductor wafers includes a slicing process for obtaining wafers of thin disc shape by slicing a single crystal ingot; a chamfering process for chamfering a peripheral edge portion of each wafer produced by the slicing process in order to prevent cracking or breakage of the wafer; a lapping process for flattening the surface of each chamfered wafer; an etching process for removing mechanical damages remaining on the chamfered and lapped wafers; a mirror polishing process for mirror polishing the surfaces of the etched wafers; and a cleaning process for cleaning the polished wafers to remove the polishing agent and foreign substances attached thereon.
The polishing process generally uses a silica containing polishing agent which has fine SiO.sub.2 particles colloidally dispersed in an alkaline solution having a pH value approximately in a range of 9 to 12, wherein wafers are polished by a combination of a mechanical action of SiO.sub.2 and a chemical action of the alkaline solution which chemically etches the silicon wafers.
The alkaline silica containing polishing agent, however, includes metal impurities. Such metal impurities included in the silica containing polishing agent may be nickel, chrome, iron, copper and so on.
Such metal impurities are included in the silica containing polishing agent because natural quartz is used as a raw material for SiO.sub.2. To increase the purity of the silica containing polishing agent, synthetic quartz is generally used instead as a raw material therefor, however, this disadvantageously increases the cost as compared with natural quartz.
Thus, measures have been taken to increase the purity of the silica containing polishing agent using a cation exchanger resin in course of manufacturing SiO.sub.2 from natural quartz. The use of the cation exchanger resin enables SiO.sub.2 to be manufactured at a lower cost than using synthetic quartz and in a purity higher than SiO.sub.2 directly manufactured from natural quartz.
It has been revealed however that contamination may occur seemingly due to copper and/or nickel even using a silica containing polishing agent using SiO.sub.2 manufactured as mentioned above.
In a continuous consideration made by the present inventors on the polishing of semiconductor silicon wafers using a silica containing polishing agent including such metal impurities, it has been revealed that metals existing in the silica containing polishing agent, particularly, copper and nickel deeply diffuse into wafers during a polishing process to deteriorate the quality of wafers and significantly degrade the characteristics of semiconductor devices subsequently formed using the wafers. However, it has been also revealed that heavy metals other than copper and nickel can be removed by cleaning the wafers after polishing so that they are not critical contaminants within wafers.
Stated another way, it has been revealed that while the concentrations of copper and nickel must be reduced as low as possible, the foregoing processing using only the cation exchanger resin is not sufficient, and other measures must be taken for increasing the purity of the polishing agent. However, measures taken for further increasing the purity cause a problem that an increase in the manufacturing cost of the silica containing polishing agent is necessarily associated therewith.
The present inventors in view of the problems inherent to the foregoing prior art, thought it necessary to clarify the concentrations of copper and nickel allowable in a silica containing polishing agent and found, as a result of diligently studying the relationship between the concentrations of heavy metals in the silica containing polishing agent and metal contamination during a polishing process, that wafers can be prevented from being contaminated by copper and nickel if they are polished in a controlled manner such that the concentrations of heavy metals, particularly copper and nickel, in the silica containing polishing agent are reduced to 1 ppb or less, and that the metal contamination can be more effectively prevented by polishing wafers while maintaining the pH value of the silica containing polishing agent at or below 11, thus reaching the present invention.